The goal of our studies is to better understand the mechanisms by which tau transmission and neuronal hyperexcitability drive the progression of neurodegeneration in Alzheimer's Disease (AD) Related Dementias (ADRD) that include Down Syndrome (DS) caused by trisomy of chromosome 21 (Ts21). Our studies are in line with the goals of RFA PAR-18-706 because we will focus on understanding ?mechanisms underlying selective cell sensitivity to tau cell-to-cell spread in circuitry.? The foundation of our studies is that tau is a prime culprit in pathogenesis of ADRDs such as DS because tau is a major component of neurofibrillary tangles in DS brains and polymorphisms in the tau gene MAPT increase risk of AD and AD in Ts21 patients. Neuropathology in ADRD follows a temporal and spatial progression in brain, and there is growing evidence that the progressive neurodegeneration in ADRD is due to transsynaptic spread of tau via connected neurons. In addition to tau, neuronal hyperexcitability can occur at early stages of ADRD, and some have proposed that network dysfunction may be a driving force for neurodegeneration in ADRDs. We will investigate if altered neuronal activity due to overexpression of amyloid precursor protein (APP) caused by Ts21 contributes to the transsynaptic spread of tau, increasing the rate of neurodegeneration. We will use a multidisciplinary approach to study how APP overexpression found in ADRDs affects transmission of tau and impacts human brain circuitry. We will co-culture organoids and transplant human iPSC-derived cortical neurons from ADRD patients (tauP301L or Ts21) with control human cortical organoids or human fetal cortical slices and monitor the impact on neuronal circuits using a novel time lapse, imaging platform, four-dimensional robotic microscopy (4DRM). 4DRM will track functional properties of individual neurons in 4D in human primary brain slices over weeks, to give insight into how APP overexpression, hyperexcitability, and tau transmission interact to cause neurodegeneration. The ability to monitor neuronal circuitry in organoids and human brain slices longitudinally is innovative and critical in studying the slow degenerative processes that may be induced by tau transmission. We will use 4DRM in combination with multivariate Cox proportional hazards analysis to determine if network hyperexcitability and tau aggregation is detrimental, beneficial or inconsequential towards neuronal survival. To complement these in vitro studies, we developed an in vivo whole animal model of tauopathy in which transgenic zebrafish larvae overexpress human tauP301L in their nervous system and use 4DRM to monitor neuronal circuitry in the living animals. The GFP-tauP301L zebrafish model shows increased p-tau but no overt neuronal dysfunction, or neurodegeneration. When hyperexcitability is induced, the fish display sensitivity to neuronal death and behavioral signs of neurodegeneration. With this model, we will investigate the effect of hyperexcitability on aggregation of tau and mechanisms of tau transmission in vivo. By understanding mechanisms of tau transmission, we can develop strategies to block spread of neuropathology to slow the progression of AD.